Passivation and pitting corrosion of stainless steel in neutral sulphate solutions

The steel samples tested in the present work were taken from heat-exchanger tubes used for sodium sulphate production at Lake Qaroun, Fayioum, Egypt. Due to the drastic extraction conditions, these tubes are usually subject to failure due to passivity breakdown and pitting corrosion. The defective tubes have to be removed, with a resulting decrease in efficiency of the extraction process. The objective of the present work was therefore to study the electrochemical behaviour of the steel samples in the sulphate medium. Potentiodynamic polarization and AC impedance techniques were used. The effect of formation conditions, namely applied potential, temperature, and chloride-ion additions, on the stability of the passive film formed on the steel surface was investigated. The results showed that pitting corrosion commences at an [Cl-]:[SO42-] ratio of approx. 0.6, and shifts in the active direction following a logarithmic low. The maximum admissible Cl- concentration at which the steel sample is protected against corrosion was found to be equal to 0.15M. Increasing the temperature of the solution decreases the pitting potential, indicating activation of the steel sample. The total impedance, Z, of the steel electrode was found to increase with increasing formation voltage, passing through a maximum due to film thickening or change in semi-conducting properties. At higher potentials, a rapid decrease of impedance was observed, due to passive film breakdown. Based on the constant-phase-element (CPE) concept, a model for the passive film was proposed and the mechanism of the corrosion and passivation processes was discussed. An excellent fit between the experimental results and the theoretical data based on the proposed model was obtained.